The present invention relates to a composition which is capable of imparting a high degree of non-Newtonian, pseudoplasticity and fluid loss controlling properties to aqueous systems and which is stable to elevated temperature conditions. More particularly, the present invention relates to the formation of an improved water-based, clay-free drilling fluid containing the subject composition and to an improved method of drilling bore holes into subterranean formations using said drilling fluid.
In normal well drilling operations in which a well is drilled by a rotary method, the well bore hole is generally filled with a drilling fluid or mud which is circulated therein. Drilling fluids are usually pumped down through the drill stem of the rotary rig, circulated around the drill bit and returned to the surface through the annular passage between the drill stem and well wall. These drilling fluids perform a number of functions including lubricating the drill stem and bit, cooling the bit, carrying the cuttings from the bit up the bore hole to the surface where the cuttings may be separated and disposed of, and providing a hydrostatic head against the walls of the well to contain downhole geopressure.
A primary requisite of a satisfactory drilling fluid is its ability to readily circulate and flow, that is, to have low viscosity, under the high shear conditions which are present in the environs of the drill bit while, at the same time, being capable of having and maintaining sufficient viscosity to be capable of efficiently carrying the bit cuttings to the surface and maintaining in suspension, any other solid components of the drilling fluid.
The drilling fluid must also be capable of inhibiting the amount of fluid, normally water, which is lost into the porous strata through which the bore hole traverses. The loss of fluid causes the formation and build-up of a cake deposit which, after a period of time, can cause sticking of the drill pipe and stoppage of the drilling operation. The drilling fluid must, therefore, be of a nature which permits minimum loss into the porous strata. Agents which impart such property are conventionally termed "water loss controllers" or "fluid loss controllers".
The drilling fluid must also be stable and functional after subjection to elevated temperature conditions. In addition to the heat generated by the frictional forces of the drill bit, it is well known that the temperatures encountered in the bore hole are substantially above that found at the earth's surface. The deeper the bore hole is, the higher the temperature encountered. Drilling to greater depths has become common in today's quest for discovering new reserves. A general rule of thumb indicates that for each 10.degree. to 20.degree. F. increase in temperature stability one can use the same fluid to drill an additional 1000 ft. It is, therefore, desired to produce a composition capable of exhibiting stability and desired functionality at the elevated temperatures commonly encountered in deep drilling operations.
It is a widely held and accepted theory that the viscosities suitable for creating a particle carrying capacity in the fluid can be achieved with the drilling fluid having pseudoplastic properties. For example, the drilling fluid must be capable of having a low viscosity under the high shear rates such as encountered at the drill bit, yet have the ability to increase in viscosity (and, therefore, particle holding power) under decreasing shear rates encountered in its upward movement through the annulus.
In order to obtain the requisite pseudoplastic properties, it has been thought desirable to use clay or colloidal clay bodies such as sodium bentonite. As a result, the drilling fluids have been usually referred to as "muds". The clay-based drilling fluids are, however, highly unstable when they come in contact with various salts found in drilled earth formations.
Materials which have come into expanding use to impart rheological properties to drilling compositions are xanthan gums such as are described in U.S. Pat. Nos. 3,198,268; 3,208,526; 3,251,147; 3,243,000; 3,307,016; 3,319,715 and 3,988,246. These materials have been found to cause aqueous solutions, such as drilling fluids, to exhibit pseudoplastic properties under varying low shear rates. However, these materials, whether used alone or in combination with other additives, present the problem of being irreversibly degraded by the elevated temperatures often encountered during conventional drilling operations.
Prior utilization of hydroxides or hydrated metal oxides of amphoteric metals in well treating fluids have involved properties distinctly different from the properties required for a drilling fluid as described herein. For example, U.S. Pat. Nos. 3,614,985 and 3,815,681 describe a process for plugging a subterranean reservoir by permeating its pores with a solution containing an amphoteric metal salt and a pH increasing reactant to cause precipitation in the pores. U.S. Pat. No. 3,603,399 describes a process for treating a water sensitive formation by permeating its pores with a hydroxy-aluminum solution which is a clear and relatively non-viscous solution. In each of such prior well treating processes, it has been important that the solution have relatively low viscosity and high fluid loss to ensure that the solution penetrates into the matrix or pores of the reservoir. Such fluids would not be suitable as a drilling mud. U.S. Pat. No. 3,860,070 describes a well completion or fracturing fluid containing an amphoteric metal salt and a base in a ratio to make the final solution strongly acidic in order to form a thickened fluid suitable as a fracturing fluid. Such fluids cannot be used satisfactorily in a drilling operation due to their corrosive nature with respect to the metal drilling equipment.
The viscosity of a drilling fluid has been relied upon as a mode of aiding in fluid loss control with little success especially when drilling into and through porous substrates. Various agents have been added to enhance the fluid loss properties of the mud. For example, U.S. Pat. No. 3,032,498 describes a cyanoethylated starch as a water loss controller to be used in combination with a clay-based mud. U.S. Pat. Nos. 3,988,246 describes an esterified or etherified starch as a water loss controlling agent which is compatible with a xanthan gum based drilling mud. Other agents as well as the starches mentioned above which have been employed in clay free muds to control fluid loss are generally found to be unstable to the temperature conditions encountered in deep well drilling.
There is a general need for a composition which is capable of imparting both pseudoplastic and water loss controlling properties to aqueous compositions while being stable to varying conditions and elevated temperatures commonly encountered in deep drilling operations.